Breast cancer is the most common type of cancer and a leading cause of death among Western women. Metastasis, a process in which primary tumor cells spread to distant sites of the body to form secondary tumors, is the primary cause of death for breast cancer patients. Most breast cancers (75%) express the estrogen receptor (ER), and both experimental and clinical evidences support key roles for ER in breast cancer metastasis. ER? antagonists such as tamoxifen have been used in ER?-positive breast cancer patients (~70% of all patients) for breast cancer prevention and treatment. Unfortunately, up to half of all ER-positive tumors either do not respond to these endocrine therapies or, after initial successful treatment, the tumors recur as endocrine-resistant breast cancer. Further, tamoxifen resistant breast cancer is known to become more aggressive and metastasize to other organs. Our recently studies supported ER coactivator MED1 as a key mediator in both tamoxifen-resistance and metastasis of human breast cancer. Importantly, MED1 overexpresses in about 40-60% of human breast cancers and the overexpression of MED1 highly correlates with poor survival of breast cancer patients. Interestingly, a recent study discovered an increased frequency of a MED1 mutation in circulating tumor cells in breast cancer patients following treatments. However, the regulation and molecular mechanisms underlying MED1 overexpression and mutation in these processes still remain poorly defined. Through both bioinformatic analyses and experimental validation, we have provided strong preliminary studies supporting that miR-205 plays an important role in regulating both MED1 expression and activation in breast cancer metastasis and resistance. Furthermore, we found that this clinically relevant MED1 mutation is hyperactive in promoting breast cancer cell metastasis and treatment resistance, as well as the expression of another class of newly identified small RNA called enhancer RNAs. Here, we will elucidate the role and molecular mechanism underlying MED1 regulation and functions in these processes using a combination of in vitro molecular biological studies, in vivo orthotopic and PDX mouse models, immuno- staining of human patient samples and RNA nanotechnology. We have assembled a strong basic and translational research team with relevant expertise to pursue the following specific aims: 1): Determine the role of the miR-205/ErbB3/ MED1 axis in tamoxifen resistance of human breast cancer; 2): Investigate the mechanisms of MED1 regulation of enhancer RNAs in breast cancer metastasis; 3): Determine the efficacy of targeting small RNAs on breast cancer therapy resistance and metastasis. Completion of this study will likely to fill a key knowledge gap in understanding novel small RNA regulatory circuits in regulating breast cancer therapy resistance and metastasis, and to make an important positive impact in providing potential novel targets for their treatment.